Concrete is an artificial material similar to stone that is used for many different
structural purposes. It is made by mixing several different coarse aggregates
such as sand and pebbles with water and cement and then allowing it to harden
by hydration. Hydration causes crystals to form that interlock and bind together.
According to the Brooklyn Public Library, concrete was first made in 500 BC and
can last up to 50,000 years. Concrete is one of the most important building materials.
From sidewalks to skyscrapers, we see concrete everyday everywhere. There are
many different kinds of concrete. Reinforced concrete is strengthened by steel.
This is done by casting concrete around steel rods or bars and most large structures
such as bridges need this extra strong concrete. Prestressed concrete is made
by casting concrete around steel cables stretched by hydraulic jacks. After the
concrete hardens the jacks are released and the cables compress the concrete.
Concrete when compressed is the strongest. This type of concrete is used for floors
and roofs as well as other things. Precast concrete is cast and hardened before
being used for construction. Precasting makes it possible to produce a mass number
of concrete building materials. Nearly all of prestressed concrete is precast
as are concrete blocks. Concrete blocks are made in various weights and sizes
and they are used to make about two-thirds of masonry walls in the US. Engineers
have designed kinds of concrete for certain uses. Air-entrained concrete is good
in harsh weather and is used for roads and airport runways. High-early-strength
Concrete is used in hurried jobs and cold weather because it hardens quicker than
ordinary concrete. Lightweight concrete weighs less than any other concrete because
it is made from pumice: a naturally light mineral.

"Typically, concrete has a density of 150 pounds per cubic foot, which means that a block of concrete that is one foot wide, one foot long, and one foot high would weigh 150 pounds. Water has the density of only 62.4 pounds per cubic foot."